The present invention relates to radio transceiver circuit technology, and more particularly to a technique to eliminate local oscillator signal phase drift between two radio transceiver integrated circuits.
A fully integrated multiple-input multiple-output (MIMO) radio frequency transceiver has been developed and is disclosed in commonly assigned co-pending U.S. application Ser. No. 10/065,388, filed Oct. 11, 2002, entitled “Multiple-Input Multiple-Output Radio Transceiver,” the entirety of which is incorporated herein by reference. This MIMO radio frequency integrated circuit (MIMO RFIC) may include two or more transmitters and two or more receivers in the same IC to simultaneously transmit multiple signals and/or to simultaneously receive multiple signals.
One application of the MIMO RFIC is in communicating between two devices using a MIMO radio algorithm. Examples of MIMO radio algorithms are disclosed in commonly assigned and co-pending U.S. patent application Ser. No. 10/174,689, filed Jun. 19, 2002, and entitled “System and Method for Antenna Diversity Using Equal Power Joint Maximal Ratio Combining;” U.S. patent application Ser. No. 10/174,728, filed Jun. 19, 2002, and entitled “System and Method for Antenna Diversity Using Joint Maximal Ratio Combining,” and U.S. patent application Ser. No. 10/064,082, filed Jul. 18, 2002 and entitled “System and Method for Joint Maximal Ratio Combining Using Time-Domain Based Signal Processing,” the entirety of all of which are incorporated herein by reference. For optimal performance of these MIMO radio algorithms, the individual transmitters and receivers should be phase-coherent in each RFIC. Consequently, the local oscillator signal used to drive each transmitter and each receiver in each RFIC must be phase coherent (same phase). This is a manageable problem internal to the RFIC since each transmitter and each receiver are driven by the same local oscillator signal.
When it is desired to exploit the benefits of the MIMO radio algorithms by increasing the number of antenna paths to, for example, four (4) at a device, two MIMO RFICs each having two transmitters and two receivers may be used cooperatively with a common baseband signal processor. In this configuration, phase coherency of the combined transmitters and combined receivers must be achieved across two separate RFICs in order to achieve the maximum performance benefits of certain MIMO radio algorithms. No techniques to solve this problem are heretofore known.
Each MIMO RFIC will have its own frequency synthesizer to generate local oscillator signals. Due to static phase error variations between the synthesizers of both radios, the relative phase between the two radio paths will drift with the variation in the static phase errors of the two synthesizers. This static phase error variation can be due to propagation delay and/or leakage current variation due to temperature gradients. In addition, the synthesizers in each RFIC could have different low frequency offset noise characteristics, resulting in different low frequency wander.
Accordingly, a technique is needed to achieve phase as well as frequency coherency between two RFICs that are used in a cooperative manner, such as with a common baseband signal processor.